The present invention relates generally to a memory unit of the type used as an auxiliary storage device for a computer. More particularly, the invention is concerned with a memory unit having an improved capability of protecting data which is stored in an incorporated memory device during a data read/write operation. The present invention is also concerned with a method of protecting data stored in the memory unit.
As auxiliary storage equipment for a computer, word processor, built-in type machine control computer or the like, there is employed a semiconductor memory unit, including a single or a plurality of semiconductor memory devices, such as a RAM (random access memory), ROM (read-only memory) or the like, which are housed in a case. When a RAM is used as the memory unit, a memory back-up power supply cell is also contained in the memory unit. This type of semiconductor memory unit can be detachably connected (slot-in/slot-out) to a control terminal of the computer by means of a connector so that a data reading/writing operation can be performed on the memory unit by the control terminal. The memory unit is so designed that data written in the memory unit can be maintained even when the memory unit is disconnected from the control terminal.
In this type semiconductor memory unit, it is apparently desirable to protect the data written in the memory from inadvertent erasure and from erroneous rewriting. Further, there are certain applications where it is required to inhibit data from being read out by a third party in order to ensure the confidentiality of the written data.
Under the circumstances, there has been known heretofore a technique according to which an externally manipulatable lock piece or member capable of assuming selectively a first or second state (first or second position) is provided as a part of the case of the semiconductor memory unit. When the semiconductor memory unit is inserted in a control terminal, the latter detects the state (or position) of the manipulatable piece to permit (enable) the data writing operation to the memory unit when the piece is in the first state, while inhibiting the data writing in case the piece is in the second state.
The known data protecting technique mentioned above suffers from a problem in that, because of the external manipulatability of the piece to be set to the first or second state, there may arise the unwanted possibility that the data stored in the semiconductor memory unit could be inadvertently erased or rewritten due to erroneous manipulation of the lock piece.
Further, with the manipulatable lock piece mentioned above, it is impossible to ensure the confidentiality of the stored data by inhibiting with certainty a third party from an attempt to read out the data, because any one can read out the data by correspondingly manipulating the lock piece.
On the other hand, a so-called IC card is a typical type of the memory unit for which a data write/read operation is prohibited. This IC card includes a microprocessor and a semiconductor memory device incorporated integrally in the body of the card, and data referred to as password or ciphered number is previously stored, so that the data write/read operation is permitted only when a password or ciphered number inputted externally coincides with the previously stored password.
Such an IC card is intended to be used as a bankbook or a nominal list, by way of example. To this end, the IC card is provided with an additional function to classify and rearrange the data in addition to the mere data storing function with a view to realizing the storage of data in accordance with preestablished rules of data arrangement. This function can be accomplished by providing the microprocessor on the card.
In contrast, the semiconductor memory unit of concern is a portable auxiliary memory or storage device for a computer which has a large storage capacity and is required only to have the data storage function, since the storage capacity of the semiconductor memory unit can not be utilized effectively if the data classification and rearrangement functions are employed, as in the case of the IC card. Of course, a microprocessor may be incorporated in the semiconductor memory unit for enabling or inhibiting data write/read operations with the aid of a password or ciphered number. In that case, however, the function of the microprocessor is not made use of fully, which is a disadvantage from the economical standpoint. Beside, in light of the requirement imposed on the semiconductor memory unit that it should be compact and have a large storage capacity, incorporation of the microprocessor should be avoided since otherwise the semiconductor memory unit could not be realized in a compact form with a large storage capacity because the microprocessor occupies a large proportion of available space in the semiconductor memory unit. Additionally, the microprocessor is indispensably accompanied by a program for operating it as well as a memory for storing the program, giving rise to a problem that the circuit scale of the semiconductor unit is enlarged, involving increased expenditure. For these reasons, it is undesirable to incorporate the microprocessor in the semiconductor memory unit.